Use of spectrometry for analysis of liquid mixtures, such as fuel, is known in the art. For example, U.S. Pat. No. 5,262,645 describes a method for measuring alcohol concentration in an alcohol/gasoline mixture by passing light through the mixture at a first wavelength that is strongly absorbed by the alcohol and weakly absorbed by the gasoline and at a second wavelength that is weakly absorbed by the alcohol and strongly absorbed by the gasoline. An absorbance ratio based upon absorbance measurements taken of the mixture at both wavelengths is computed. From this ratio and fuel temperature the concentration of alcohol in the fuel is determined.
There are two major difficulties associated with measuring additive concentrations in fuels using infrared (IR) spectroscopy. A fuel additive's spectral signature at its characteristic transmission peak can be extremely small. As a result, concentration of a fuel additive in a mixture can be at or below the detection threshold. Also, the background spectral variation of the fuel mixture is typically large compared to the spectral signature of the fuel additive, leading to interference in the spectral signature of the fuel additive.